Production of flat punchings



Patented June 27, 1933 UNITED STATES PATENT OFFICE JOHN C. GRANFIELD, OF PITTSFIELD, MASSACHUSETTS, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK PRODUCTION OF FLAT PUNCHINGS No Drawing.

The present invention relates to a method for annealing sheet steel and more particularly to a method for producing flat, annealed sheet metal punchings. It is frequently desirable to employ relatively large silicon steel sheets as laminations in cores of electric apparatus such as transformers, dynamos and the like. For best results these laminations should be perfectly flat when ready for assembly. In the usual annealing process, relatively thin punchings are stacked in the furnace to a height of 18 inches or 3 feet on either a sand base or a relatively thick rolled steel base and heated to the desired annealing temperature. When supported in this manner, however, and heated, the annealed sheets, even when relatively small in area, are usually in a warped condition when ready for assembly and the tendency to become warped or distorted is increased when the area of the sheets to be annealed is increased.

It is one of the objects of the present invention to provide a method for producing annealed, relatively large, fiat steel sheets.

In carrying out my invention, I employ, instead of the usual sand or cold rolled steel base, a relatively thick supporting base plate made of alloy steel and having a composition of such character that it does not pass through the A point, i. e. the point at which the metal changes from alpha to gamma iron or from a body centered lattice crystal structure to a face centered lattice crystal structure during the annealing process.

If the supporting base plate is originally perfectl fiat but made of material which passes t rough the A, point during theannealing process, the repeated heating and cooling to which such plate is subjected in successive annealing cycles causes it to contract or shrink from time to time so that it is no longer flat, but warps badly because of a tindency eventually to become spherical in s a e.

I have found that the contraction of the sup orting base plate is the principal cause of istortion in the annealed sheets and that this difliculty may be overcome by employing a supporting base plate which does not pass through the A point during the anneal of the sheet steel. Various alloys exist which have this characteristic. For example, silicon steel having a content of 2% or more of silicon satisfies this condition and remains perfectly flat after repeated heating and cooling Application filed July 7, 1930. Serial No. 466,045.

during successive annealing cycles applied to silicon steel punchings.

A silicon steel base plate having a silicon content of about 2 gives entirely satisfactory results. Steel having a higher silicon content may be employed if desired. However, steel with a high silicon content is very brittle andlthis brittleness increases as the silicon content is increased. It is preferable therefore to employ a silicon steel supporting plate having a relatively low silicon content as indicated above.

Ordinarily, the supporting plate may vary considerably in thickness. I have found however, that a thickness of about 1 to 2 inches gives satisfactory results. Instead of employing merely a silicon steel supporting plate, I may employ in addition thereto several silicon steel plates which may be interleaved with the sheets to be annealed. In the latter case, I prefer to employ silicon plates about one-half inch thick, eight or ten plates of such thickness being employed when the annealed stack is about three feet high.

As an additional aid in carrying out my invention, I prefer to employ a hydrogen or non-oxygenous atmosphere to prevent oxidation of the edges of the annealed sheets during the annealing process. Such oxidation ma cause the annealed sheets to stick toget er with consequent distortion thereof.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. The method for producing annealed fiat metal sheets which comprises stacking the sheets on a relatively thick metal plate, said metal plate consisting of silicon steel, the silicon content of said steel being at least 2%, and heating said sheets to their annealing temperature, said plate and sheets having substantially the same coefficient of expansion.

2. The method for producing annealed flat silicon steel punchings which comprises stacking the punchings on a relatively thick metal plate, said metal plate consisting of silicon steel the silicon content of said steel being at least 2%, and heating said punchings to their annealing temperature in a reducing atmosphere.

In witness whereof, I have hereunto set my hand this 2d day of July, 1930.

JOHN C. GRANFIELD. 

